1. Field of Invention
The present invention relates to a liquid crystal device, a projection type display device and a method of manufacturing the liquid crystal device. Particularly, the present invention relates to a light shielding structure in a liquid crystal device comprising a thin film transistor (referred to as xe2x80x9cTFTxe2x80x9d hereinafter) as a pixel switching element.
2. Description of Related Art
For a conventional active matrix driving system liquid crystal device a configuration is brought into practical use in which pixel electrodes are formed in a matrix on a glass substrate and a pixel switching TFT comprising a semiconductor layer of an amorphous silicon film or a polysilicon film is formed in correspondence with each of the pixel electrodes so that a voltage is applied to each pixel electrode through the TFT to drive a liquid crystal. In a liquid crystal device comprising pixel switching polysilicon TFT, TFT""S for driving circuits comprising peripheral driving circuits such as a shift register circuit, etc. for driving and controlling a screen display portion can be formed in substantially the same process as the pixel switching TFT, and thus the liquid crystal device attracts attention because it is suitable for high integration.
In such an active matrix system liquid crystal device, in order to achieve high definition of the display, a light shielding film referred to as a black matrix (or black stripes) or the like and made of a chromium film, aluminum film, or the like is formed on an opposite substrate. The light shielding film is also formed to overlap with the pixel switching TFT so as to prevent the light incident on the opposite substrate from entering the channel region and junctions of the pixel switching TFT and prevent a leak current from flowing through the pixel switching TFT. (This light shielding film is referred to as xe2x80x9cthird shielding film hereinafter.)
However, the leak current due to light is not only due to the light incident on the opposite substrate side but also irradiation of the channel region of the pixel switching TFT with the light reflected by a polarizing plate or the like disposed on the back side of the liquid crystal device substrate.
As a method of preventing the leak current due to such reflected light (return light), Japanese Examined Patent Publication No. 3-52611 discloses an invention in which a light shielding film is also provided on the lower layer side of the channel region of the pixel switching TFT. However, in the disclosed invention, the potential of the light shielding film is not fixed, and there is thus a problem in which TFT characteristics vary or deteriorate due to the parasitic capacitance between the semiconductor layer of the TFT and the light shielding film.
On the other hand, the peripheral driving circuits are increasingly demanded to be highly integrated with an increase in the number of pixels and miniaturization of an electronic apparatus containing the liquid crystal device. Particularly, in a liquid crystal device containing the peripheral driving circuits provided on the same substrate, as a technology for achieving high integration of circuits, a multilayer wiring technology is used in which wiring is provided by forming metallic films of aluminum or the like in multiple layers with insulating films between the respective layers. However, this technology has a problem in that the number of steps and production cost are increased by forming the multilayer wiring structure.
Also, as the speed of the operation frequency of the active matrix driving system liquid crystal device increases, attempts are made to improve the quality of the semiconductor film by employing a SOI technology, a recrystallization technology using laser annealing, or the like in order to improve TFT characteristics. However, such a method of improving TFT characteristics has the problem of increasing the variations in characteristics and the problem of complicating the manufacturing process.
Accordingly, an object of the present invention is to provide a technology for suppressing a leak current due to the influence of the light reflected by a polarizing plate or the like in pixel switching TFTs of a liquid crystal device and a projection type display device comprising the liquid crystal device, to stabilize the characteristics of the pixel switching TFTs.
Another object of the present invention is to provide a technology for achieving high integration of driving circuits provided in the periphery of a display region in a liquid crystal device without increasing the number of the steps of the manufacturing process.
A further object of the present invention is to provide a technology for improving TFT characteristics in a liquid crystal device without increasing the number of the steps of the manufacturing process.
In order to solve the above problems, the present invention provides a liquid crystal device comprising a liquid crystal device substrate including a display region in which pixels are formed in a matrix by a plurality of data lines and a plurality of scanning lines, peripheral driving circuits connected to at least one of the data lines and the scanning lines on the outside of the display region, and a plurality of thin film transistors connected to the data lines and the scanning lines; and a liquid crystal held between the liquid crystal device substrate and an opposite substrate;
wherein the liquid crystal device substrate has a first conductive light shielding film provided on at least the lower layer side of the channel regions of the thin film transistors so that the light shielding film and the channel regions overlap each other with an interlayer insulation film therebetween, and a constant voltage is applied to the first light shielding film.
In the liquid crystal device in accordance with the present invention, since the first light shielding film is formed to overlap with the channel regions of the thin film transistors, i.e., the pixel switching TFTs, connected to the data lines and the scanning lines, even if light is reflected from the back side of the liquid crystal device substrate, the reflected light does not enter the channel regions of the pixel switching TFTs. Therefore, no leak current occurs in the pixel switching TFTs due to the light reflected from the back side of the liquid crystal device substrate. In addition, since the potential of the first light shielding film is fixed at the constant voltage power source on the low-potential side of a scanning line driving circuit, the TFT characteristics neither change nor deteriorate due to the influence of the parasitic capacitance between the semiconductor layers of the TFTs and the first light shielding layer.
In the present invention, in order to apply a constant voltage to the first light shielding film, the first light shielding film may comprise channel shielding portions which respectively overlap with the channel regions, and wiring portions extended from the channel shielding portions in order to apply a constant voltage to the channel shielding portions.
In this case, the wiring portions of the first light shielding film are respectively extended from the channel shielding portions to the outside of the display region along the signal lines of at least either of the scanning lines and the data lines, and are connected to constant potential wiring formed between layers different from the first light shielding film at least through contact holes of the interlayer insulation film in the outside of the display region.
In some cases, the wiring portions of the first light shielding film are respectively extended from the channel shielding portions to the outside of the display region along both signal lines of the scanning lines and the data lines, and are connected to constant potential wiring formed between layers different from the first light shielding film at least through contact holes of the interlayer insulation film on the outside of the display region.
In the present invention, each of the wiring portions of the first light shielding layers is connected to the constant potential wiring through the contact hole of the interlayer insulating film on the outside of the display region.
When one side of each of the wiring portions of the first light shielding film is connected to the constant potential wiring through the contact hole of the interlayer insulating film, a constant voltage can be applied to the first light shielding film.
On the other hand, when both ends of each of the wiring portions of the first light shielding film are connected to the constant potential wiring through the contact holes of the interlayer insulating film, even if the wiring portions of the first light shielding film are disconnected at an intermediate position thereof, a constant potential is supplied to the wiring portions of the first light shielding film from the constant potential wiring. Therefore, the wiring portions of the first light shielding film comprise redundant wiring and thus exhibits high reliability.
In the present invention, the wiring portions of the first light shielding film comprise branches respectively extended from the channel shielding portions to the outside of the display region along the signal lines of at least either of the scanning line and the data lines, and a trunk to which the branches are connected on the outside of the display region, the trunk being preferably connected to the constant potential wiring through the contact holes of the interlayer insulation film. This configuration eliminates the need to connect the first light shielding film and the constant potential wiring for each of the branches, and thus the trunk and the constant potential wiring may be connected. Therefore, the trunk may be lead to any desired position without wiring and connected to the constant potential wiring. In the connection between the first light shielding film and the constant potential wiring, wet etching for forming the contact holes easily produces cracks in the interlayer insulation film due to penetration of an etching solution. However, the present invention has the advantage that the trunk can be lead to any desired position, and a place where the racks might occur is limited to a safe position. The present invention also has the advantage that since the place where the cracks might occur is minimized by connecting the trunk and the constant potential wiring in order to connect the first light shielding film and the constant potential wiring, the reliability is high.
In this case, a constant voltage can be applied to the first light shielding film by connecting one end of each of the branches to the trunk.
On the other hand, when both ends of each of the branches are connected to the trunks, even if a branch is disconnected at an intermediate position thereof, a constant voltage is applied to the wiring portion of the first light shielding film from the trunks. Therefore, the wiring portion of the first light shielding film comprises redundant wiring, and thus exhibits high reliability.
In the present invention, the first light shielding film preferably is connected to capacitance wiring which overlaps with the drain regions of the pixel switching TFTs to form the storage capacitor, through at least the contact holes of the interlayer insulation film. Also the first light shielding film preferably overlaps with the drain regions of the pixel switching TFTs through the interlayer insulation film to form the storage capacitor. This configuration eliminates the need to lead each capacitance wiring into the scanning line driving circuit in order to apply a constant voltage, and thus facilitates the layout for introducing a large scale circuit into the scanning line driving circuit.
In the present invention, the constant potential wiring is connected to a power source line for supplying a power source on the low-potential side to the peripheral driving circuits, a power source line for supplying a counter electrode potential to the counter electrode of the opposite substrate from the liquid crystal device substrate through transfer materials, or a power source line for supplying a ground potential to the peripheral driving circuits.
In the present invention, at least one of the liquid crystal device substrate and the opposite substrate preferably comprises a light shielding film for partitioning the display screen, which is provided to surround the display region.
In the present invention, the liquid crystal device substrate preferably comprises a second light shielding film which is provided on the upper layer side of the channel regions of the pixel switching TFTs to cover the channel regions. In this case, as the second light shielding film, for example, the data lines can be used. The second light shielding film is preferably formed to cover not only the channel regions but also the first light shielding film formed below at least the channel regions through the interlayer insulation film so as to prevent incident light from being reflected by the surface of the first light shielding film and applied to the channel regions of the pixel switching TFTs. This configuration permits a decrease in the leak current due to light in the TFTs.
In the present invention, the peripheral driving circuits include P channel type driving circuit TFTs and N channel type driving circuit TFTs, the P channel type and N channel type driving circuit TFTs being preferably formed by the same process as that for producing the pixel switching TFTs. This configuration limits the number of the layers in multilayer wiring, and thus, in the peripheral driving circuits, the conductive film formed at the same time as the first light shielding film is preferably effectively used as an wiring layer.
In the present invention, the wiring layer comprising the conductive film formed at the same time as the first light shielding film is preferably connected to the gate electrodes of the driving circuit TFTs at least through the contact holes of the interlayer insulation film, and overlaps with the channel regions of the driving circuit TFTs with an area smaller than the area of the gate electrodes of the driving circuit TFTs on the lower layer side of the channel regions through the interlayer insulation film.
In the present invention, the wiring layer comprising the conductive film formed at the same time as the first light shielding film is preferably connected to the source electrodes of the driving circuit TFTs at least through the contact holes of the interlayer insulation film, and overlaps with the channel regions of the driving circuit TFTs on the lower layer side thereof.
In the present invention, the first light shielding film preferably comprises an opaque conductive film such as a metallic film made of tungsten, titanium, chromium, tantalum, molybdenum, or the like, or a metal alloy film made of meal silicide or the like, which contains any one of these metals. Such a metallic film or metal alloy film which has high light shielding ability and electrical conductivity functions as a light shielding layer for the light reflected from the back side of the liquid crystal device substrate.
In the present invention, the opposite substrate preferably comprises a third light shielding film formed corresponding to the pixels. In this case, the third light shielding film is preferably formed to cover at least the first light shielding film.
In the present invention, the opposite substrate preferably comprises microlenses formed in a matrix corresponding to the pixels. This construction permits the microlenses to converge light on a predetermined region of the liquid crystal device substrate, and thus permits high-quality display even if the third light shielding film is omitted from the counter substrate. The liquid crystal device of the present invention prevents irradiation of the channel regions of the pixel switching TFTs even if the light converged by the microlenses is reflected by the back side of the liquid crystal device substrate, and thus no leak current flows through the TFTs due to light.
Since the leak current due to light is suppressed in the TFTs, the liquid crystal device of the present invention is preferably used as a light valve for a projection display device which is irradiated with strong light. In such a projection type display device, the light emitted from a light source is modulated by the liquid crystal device of the present invention, and the modulated light is enlarged and projected by projection optical means.